Relative term; transition with thorntree savanna and other grassland types
Arid regions where potential evaporation is very much higher than annual precipitation
Distinction between subtropical and temperate (marked cool/cold winter) deserts - must exclude the Great Basin of North America, Gobi and other central Asiatic deserts. The cold winter period typical of arid regions in temperate zone is lacking in subtropical desert region.
Subtropical desert has less then 200mm rain and potential evapotranspiration greater than 2000mm
II. Locationand types of deserts
Subtropical deserts lie between 15 and 30 latitude; centered on Tropics of Cancer and Capricorn; here is where the air carried up from intertropical convergence zone subsides as subtropical highs; adiabatic warming as the air descends (at the dry rate!) coupled with intense solar radiation in non-cloudy skies results in the oppressive hot and dry climate.
In all deserts (except fog deserts) the air is very dry. Deserts are subdivided into semiarid, arid, extremely arid, rainless deserts (Atacama desert in Chile, 18-28 S; central Sahara desert) based on total precipitation.
Subdivided into subzonobiomes by Walter based on when the sparse precipitation falls. For example:
(1) Sonoran desert: 2 rainy seasons
(2) Mohave desert: winter rainy season
(3) inner Namib and southern Sahara deserts: summer rainy season
(4) central Australia: sparse rainfall occurring anytime
(5) Chilean-Peruvian and outer Namib deserts: only fog; cool and foggy due to cold water currents (e.g., Benguela Current is 12-16°C) and thus cold air; Namib shows 200 days of fog; show very little temperature change throughout the year; other subtropical deserts have a distinctive cool season when the sun is low
Distinctive feature of all arid regions is the large variability in amount of rain falling in different years. For example, Namib desert: average is 15mm, but ranges from 0 to 140mm (the latter once in a century). Plants respond to this seasonal precipitation pattern as well as to the unpredictability inherent in desert systems.
Together desert (andother arid) regions cover 35% of the earth's surface
Due to high evaporation of water in deserts, clay soils form the driest habitats as water is kept close to the surface by capillary action and thus prone to evaporation. Sandy soils offer better water supplies, and rocky and fissured soils provide the wettest habitat. This is perhaps counterintuitive at first.
Groundwater level and wetness of overlaying rock is critical in selection of plants that will occupy deserts. In case of deeper groundwater but clay top soil (dry), it is difficult for non-deep rooted plants to tap water sources; however, when and if top soil is wet from the surface down several years in succession, then the woody plants with deep roots can reach groundwater and survive even when the top is quite arid.
Halophytes (salt loving plants): Salt builds up in many desert systems when water is evaporated leaving behind the minerals placed in solution by water through capillary action or washed from higher elevations. Chlorine is the one element that is really retained and causes plants problems. Halophytes are plants that adapt in various ways to high salt regimes.
a. salt accumulators (usually NaCl) in vacuoles or specific organs, usually associated with succulence (Chenopodiaceae: Salicornia, Suaeda, Chenopodium, Atriplex)
b. salt excretors (Tamarix, tamarisk tree)
c. non-halophytes (Atriplex; Zygophyllaceae: Zygophyllum)
III. Desert Life Forms
Xerophytes (dry loving plants) are common:
malakophyllus xerophytes: characteristic of semi-arid regions; soft leaves that wilt under dry conditions while cell-sap concentration rises sharply; in lengthy dry periods all leaves but hair covered buds survive (Lamiaceae and Compositae)
scleophyllous xerophytes: small hard leaves and owe their rigidity to mechanical tissue; found in regions with long summer drought; reduce transpiration when water is scarce (evergreen Quercus [oaks], Olea [olive], Ilex [holly])
stenohydric xerophytes: stomates shut down at first sign of water shortage; no gas exchange and no photosynthesis, thus state of starvation (most desert plants fit in this category)
succulents: water-stroing species; the small water absorbing roots die during the dry period; distinctive component but small component
succulent leaved species: Agave, Aloe, Crassula, Sanevieria, Mesembryanthemum
succulent stems species: Euphorbia, Stapelia, cacti
succulent tubers: Asparagus, legumes
Many species cope with aridity by by-passing interaction with dry period by being ephemeral:
therophytes (annuals via seeds)
geophytes (survive under ground)
Sahara: 50% of desert species in Africa found in Sahara. These include: perennial grasses (Aristida, Stipa, Panicum); shrubs (Ephedra, Tamarix, Acacia, Zygophyllum), and trees (Phoenix dactylifera - date palm) at oases.
Southern Africa (including the Namib, Kalahari)
Welwitschia mirabilis, Euphorbia, Pachypodium
succulent Aloe and other Liliaceae, Crassulaceae, Aizoaceae, Mesembryantheumum (Aizoaceae) spp.
Conophytum and Lithops (the diminutive 'stone' plants)
41% of Autralia's 1,200 species confined to desert and desert-like areas including the 'spinifex' grasslands dominated by Triodia grass hummocks, saline soil areas with Atriplex (saltbush) and Kochia (bluebush) (both Chenopodiaceae), and 'mulga' with Acacia aneura, Eucalyptus (Myrtaceae)
C. South America
Atacama desert on Tropic of Capricorn. Fog desert to the south where only moisture is fog; Tillandsia of Bromeliaceae and cacti able to absorb this moisture
Patagonian desert furthest south (temperate desert) on leewardside of Andes
Dominant plants include Larrea divaricata (creosote bush); grasses: Stipa, Sporobolus; shrubs of Capparis, Acacia; cacti, and many annuals
D. North America (subtropical deserts only - Great Basin is a temperate desert)
Chihuahuan Desert is the most southern and subtropical of these deserts located primarily in Mexico. Therefore rains would only come when subsolar point is most northerly - i.e., summer (as with subtropical forests). Dominated by creosote bush (Larrea tridentata), tarbush (Flourensia cernua), white thorn (Acacia constricta), Agave and Yucca
Sonoran Desert is more intermediate between the other two deserts and thus has both the summer rain of the Chihuahuan Desert and winter rain period of the Mojave Desert. Dominate plants include saguaro (Cereus giganteus or Carnegiea gigantea) , organpipe (C. thurberi), chollo (Opuntia bigelovii), paloverde (Cercidium microphyllum), mesquite (Prosopsis glandulosa), ironwood (Olneya tesota), bursage (Ambrosia dumosa), ocotillo (Fouquieria splendens), creosote bush (Larrea tridentata), Mormom tea (Ephedra viridis) and lots of winter annuals
Mojave Desert is close to Mediterranean climate of California and is thus cooler and wetter in winter. Typical plants include Joshua tree (Yucca brevifolia), creosote bush, bursage, and lots of winter annuals.
IV. Evolution of DesertFloras
Although geological record suggests that arid times have existed since Devonian Period (400mya), there are contrasting ideas on desert origin
Axelrod (1958) postulated that desert flora originated in Miocene epoch (24mya of the Tertiary Period), expanded during the cold dry periods of the Pliocene (2mya at end of the Tertiary Period), and reached its present extent during Pleistocene (last 2 my in Quaternary Period)
Shmida (1985) argues that the richness and uniqueness of a desert flora reflects its size, age, and period of isolation. Likewise, Whittaker (1977) argues that the distinctive life forms in deserts argues for long periods of time
Floristic links are strong within floristic areas and weak between floristic areas. However, the two great northern hemisphere temperate deserts (Great Basin Desert and Asian deserts) are floristically similar perhaps due to Beringian land bridge until late Pleistocene.
Again as in other biomes, there is interesting interplay between vegetation and flora. Cactaceae in American deserts, Euphorbiaceae in African deserts, Aizoaceae in southern Africa, Didieriaceae in Madagascar deserts. Two genera of the gymnospermous order Gnetales (now considered conifers) are restricted to separate regions: Welwitschia in Namib desert, Ephedra in North and South American deserts.